The present invention relates to high DC current acyclic or homopolar generators and particularly to the circulation of liquid metal in the collectors thereof.
A goal of present research and development efforts is to develop smaller, ultra-high current acyclic generators of dramatically increased power density. To this end, high current density field coils, such as supercooled or superconducting field coils, are utilized to provide the requisite high density magnetic field. This coupled with dramatic increases in the peripheral velocity of the rotor can develop DC current outputs in the megamp range. That is, the high peripheral velocity affects the voltage which, when combined with the high current, results in high power density. To accommodate such high DC current magnitudes and peripheral velocities, liquid metal collectors are a virtual necessity to reliably handle current transport between the rotor and stator of the generator.
At such high currents and peripheral velocities, control of the liquid metal, typically a sodium-potassium eutectic (NaK), becomes extremely difficult due to the myriad forces acting on the liquid metal. Obviously, the liquid metal must continuously wet the rotor and stator collector surfaces and completely fill the gaps therebetween to avoid arcing and undue losses. In addition, the current carried by the liquid metal, coupled with the physical agitation thereof during high current, high velocity generator operation, generates considerable heat in the liquid metal, which must be removed if it is not to raise its resistivity and thus increases losses. Thus, it is important that the liquid metal be continuously removed from the collector regions, cooled and returned thereto in recirculating fashion, all without creating voids in the gaps between collector surfaces.
Complicating these objectives is the force exerted on the liquid metal resulting from the interaction of the generator current flowing therethrough and the magnetic field associated therewith. This outwardly directed Lorentz force tends to drive the liquid metal out of the collector gaps and is a direct function of the current magnitude. Thus, as the generator current is increased, Lorentz expulsion forces become a significant factor. In addition, the generator current coacts with the component of the generator magnetic field existing in the collector gap which is normal to the current path therethrough to develop forces driving the liquid metal in a circumferential direction opposite to the direction of rotor rotation.
In addition to the above-noted magnetohydrodynamic motoring forces acting on the liquid metal in the collector gaps, mechanical forces exerted on the liquid metal due to the high rotational velocity of the rotor must also be taken into consideration. First, there is a viscous pumping force which tends to drive the liquid metal in the same circumferential direction in which the rotor collector surface is moving. Thus, this pumping force acts in the opposite circumferential direction to the magnetohydrodynamic forces generated by the coaction of the generator current and the generator field in the collector gaps. At zero generator current, this viscous pumping force causes the liquid metal in the collector gaps to revolve circumferentially at velocity equal to one-half of the rotor peripheral velocity. As generator current is increased, so does the counteracting circumferential magnetohydrodynamic force. At some current value, circumferential motion of the liquid metal will be halted, and at higher values, the liquid metal will be driven in a direction opposite to the direction of rotor rotation. Such counter-rotation of the liquid metal significantly increase viscous drag on the rotor, resulting in higher losses.
Finally, there are the radially directed, centrifugal pumping forces acting on the liquid metal due to the rotational motion of the rotor surfaces in contact therewith.
It is seen that these liquid metal pumping or motoring forces vary with generator current and rotor velocity. Thus, it becomes extremely difficult to develop a design capable of affording the requisite control of the liquid metal over a wide range of operating conditions from zero to rated generator current and zero to rated rotor velocity.
In addition to the foregoing considerations, it would be desirable to utilize these magnetohydrodynamic and mechanical forces to reliably recirculate the liquid metal through the collector region under all operating conditions and thus avoid the added complexity and cost of an external pump to move the liquid metal in a recirculating path through the collector gaps.
It is accordingly an object of the present invention to provide an improved liquid metal collector for an acyclic generator.
An additional object is to provide a liquid metal collector of the above-character, wherein the inherent dynamic forces acting on the liquid metal during generator operation are advantageously controlled such as to achieve circulation of the liquid metal through the collector region.
Yet another object is to provide a liquid metal collector of the above-character, wherein recirculation of the liquid metal through the collector region pursuant to extracting heat therefrom is achieved virtually independently of generator current and without resort to an external recirculating pump.
A still further object is to provide a liquid metal collector of the above-character having improved efficiency and capable of handling extremely high power densities.
Another object is to provide a liquid metal collector of the above character which is efficient in design and reliable in operation over a wide range of operating conditions and over a long service life.
Other objects of the invention will in part be obvious and in part appear hereinafter.